We had an application running a business for 10 years. It started printing labels incorrectly. There was a bug in the code. The source code provided for the application was at least 5 years out of date. Documentation was null. Additionally, the variable names were TextBox1, TextBox2....etc.

I decompiled the app, documented the UI and which variable is what. Rewrote the whole app. It was quite painful but good learning experience. I would love to do another one.